It is critical to identify and characterize factors involved in obesity, so that interventions to prevent or reverse the nutrition-related negative health sequelae of obesity can be developed. To do so requires a better understanding of lean-obese differences in the mechanisms that determine human food intake. The present study will indicate whether obese individuals can respond to internal physiological cues related to the energy content of foods. The effects of volume manipulations on food and nutrient intake over a one day period will be accurately characterized and measured to determine if lean and obese subjects can compensate for energy manipulations and if volume of the oral or intragastric (IG) preloads differentially affect subsequest food choice or intake. The study will test the following hypotheses: a)Both volume and energy cues will have to be present for IG to be effective in reducing subsequent energy intake. This will be determined by comparing the energy regulatory responses to various energy-present pre-loads and no pre-loads. b) Obese individuals will be less responsive to mechanical and nutrient stimulation of the stomach than lean individuals. This will be determined by comparing energy regulatory responses after oral and IG preloads of different volumes and energy densities in lean and obese individuals.